Are we specifically talking about the '02 team that went to the ECF?
We can quibble about who was better between Pierce/Walker and Tatum/Brown but I think the main thing (and the obvious thing) is how much better the rest of the roster is on the '21 and '22 teams.
Who was the third best player on the '02 Celtics? Was it washed up Kenny Anderson averaging 9 points and 5 assists? Or Tony Battie putting up 7 points and 6 rebounds? The 3rd leading scorer on the team was actually Rodney Rogers, putting up almost 11 ppg in 27 games.
Compare them to '21: Kemba and Smart were both better than Anderson, Theis, Thompson and Timelord were all both better than Battie, and Evan Fournier, despite everything he went through as that team's late-season addition, was better than Rogers.
We can do basically the same thing with the upcoming season. Smart, Schroder, Richardson, Timelord, Horford would likely all have been the 3rd best player on the '02 team. Enes Kanter is going to be the 3rd string center next season and I think you can make an argument he's better than Tony Battie, who was the starter.
This Celtic team is miles ahead of that team of the early 00's.